SPI (samples per inch) is scanner and digital image resolution. To scan an image the scanner takes a sampling of portions of the image. The more samples it takes per inch, the closer the scan is to the original image. The higher the resolution, the higher the SPI.
PPI (pixels per inch) is the number of pixels displayed in an image. A digital image is composed of samples that your screen displays in pixels. The PPI is the display resolution not the image resolution. (Adobe Photoshop uses PPI and Corel Photo-Paint uses DPI for image resolution so it's no wonder everyone is confused.)
DPI (dots per inch) is a measure of the resolution of a printer. It properly refers to the dots of ink or toner used by an imagesetter, laser printer, or other printing device to print your text and graphics. In general, the more dots, the better and sharper the image. DPI is printer resolution.
LPI (lines per inch) refers to the way printers reproduce images, simulating continuous tone images by printing lines of halftone spots. The number of lines per inch is the LPI, sometimes also called line frequency. You can think of LPI as the halftone resolution.
To fully understand all these terms we need to know more than the proper definitions. We need to know how this translates to real world usage. The next four pages explain each term more fully and provide examples of how the terms relate to each other, to onscreen display and to printing.
It is important to remember that many of the tips, tutorials, and even software that you'll encounter (including some of the links from this feature) use SPI/PPI/DPI interchangeably. Look at the terms in context to help you decipher whether it means samples, pixels, dots, or spots.
Next > SPI: Image Resolution
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